Section V. FABRICATING CAST CHROME-BASE ALLOY FRAMEWORK 10-29. Duplicating the Master Cast a. General. The fabrication of the cast metal framework of the removable partial denture is carried out on a refractory cast which is a duplicate of the master cast and composed of chromebase alloy investment material. The master cast cannot be used because it would be destroyed during the casting process and there would be no cast upon which the prosthesis could be finished. A second duplicate of the master cast is poured in artificial stone. This duplicate is used to fabricate the denture base. Procedures used to obtain these duplicate casts are described below. (1) In producing duplicate casts it is essential that the dimensional accuracy and all minute details of the master cast be reproduced faithfully. Reversible hydrocolloid impressions are made of the master cast. From these, the refractory and artificial stone duplicate casts are poured. (2) Reversible hydrocolloid impression materials shrink considerably when cooling and solidifying. Shrinkage can cause voids in the impression and it may set up strains in the material. To prevent warpage and distortion and loss of accuracy in the duplicate casts, some provision must be made to compensate for this shrinkage. Special flasks and rings have been developed to provide for chilling the impression from the base upward and to permit additional material to be drawn into the impression from the excess in a reservoir at the top of the ring (fig 10-40). b. Preparation of Master Cast. Before the master cast is duplicated, the method of spruing must be determined. (Spruing is explained in para 10-35). If inverted spruing is to be used, a short metal sprue cone is positioned and attached to the master cast. If top spruing is to be used, a sprue cone is not indicated. To prepare the master cast for duplication it is soaked in warm water (100° to 105° F.) with the level of the water below the tissue surface of the cast. This wets the cast so that the hydrocolloid material will not stick to it and expels the air from the cast so that bubbles will not form on the surface of the mold during the pouring. It also raises the cast temperature closer to that of the fluid reversible hydrocolloid material so that the latter will flow evenly over all surfaces. c. Preparation of Reversible Hydrocolloid. (1) The material is cut into small cubes by using a knife or food chopper. The cubes are placed in a double boiler and water added in the amount recommended by the manufacturer. The material is heated slowly (205° to 210° F.) to a thin consistency, stirring occasionally. Stirring breaks up any remaining lumps of material and prevents the formation of air bubbles in the mixture. To keep from losing water, the container is kept covered when it is not being stirred. When the mass has become smooth and homogeneous, the container is removed from the heat and stirred continuously until the material has cooled to the temperature recommended by the manufacturer as proper for pouring. If the material is not stirred continuously, the hydrocolloid on the sides of the container will cool quicker than the rest of the mass, forming lumps which will spoil the pouring of the mold. (2) If a thermostatically controlled reversible hydrocolloid conditioner is used, the mix can be ready for almost instant pouring. The temperature control is set between 125° and 130° F. The mix should be stirred well before pouring. The valve is freed of any material which may have solidified in the opening after previous use. This can be done by forcing a rod of proper size through the valve with a pumping action. When the mix flows freely, the valve is closed. The material is ready for the filling of the flask. d. Pouring the Mold. To pour the mold, the master cast is removed from the water bath and any excess water is shaken or blown off. The cast is placed in the base of the duplicating flask and anchored with modeling clay. The body portion of the flask is placed on the base. The reversible hydrocolloid is poured in a slow stream allowing it to flow evenly into the rest area, interproximal areas, and other areas of fine detail. When the flask is almost filled, the lid section and reservoir are placed in position and hydrocolloid is added until the flask and reservoir are full (A, fig 10-41). e. Cooling the Mold. The mold must be cooled slowly and from the bottom of the flask (B, fig 10-41). It is placed in cold running water at a level 10 to 12 mm up on the side of the flask. It is left standing in the water 45 minutes. f. Removing Master Cast. When cooling is completed, the base of the flask is removed and with a knife, two semilunar sections of the hydrocolloid material are cut about 12 mm. deep from opposite sides of the base of the cast. A firm grip is made on the cast and it is removed from the mold with a quick snap (C, D, and E, fig 10-41). Slow dragging or rocking motions will distort the mold. Excess water is removed and the mold is inspected for accuracy of details. Two knives may also be used to remove the cast from the mold. A moderate blast of air around the cast base will loosen it before it is removed. g. Pouring the Second Mold. A second mold is made of the master cast using the procedure described in d above with the following exception. The relief pads in the denture base areas and sprue cones are removed from the cast before the mold is poured. This step requires an additional flask procedure. h. Pouring the Refractory Cast. (1) The refractory cast should be poured immediately after the master cast is removed from the hydrocolloid mold. Chromebase alloy investment powder is weighed, the water (distilled water preferred) is measured, and the materials are mixed according to the manufacturer's instructions. The investment material can be mixed either by hand or power-driven spatulator for 30 seconds. Air should not be trapped in the mix during spatulation. Excess moisture is blown from the hydrocolloid mold. (2) The investment material is vibrated into the hydrocolloid mold from one side until the level of the hole in the impression has been reached (A, fig 10-42). A medium-sized sprue cone is then inserted into the hole (B, fig 10-42). (3) A small amount of investment material is vibrated around the sprue cone to form a seal. This will prevent having to hold the cone in position while the rest of the cast is being poured. The impression should not be overfilled (C, fig 10-42). (4) The investment is allowed to set 1 hour before separating the refractory cast from the impression. With an indelible pencil a line is drawn around the cast about 6 mm. beyond the limits of the proposed pattern. The refractory cast is trimmed to this line (D, fig 10-42). i. Pouring Artificial Stone Duplicate Cast. Artificial stone is poured into the second hydrocolloid mold. The denture base material containing the denture teeth will be fabricated on this duplicate cast. j. Comparison of Casts. When the duplicate casts have been completed, the hydrocolloid is broken into small pieces for reuse. The duplicate casts are compared with the master cast to make certain that the dimensions and details have been accurately reproduced. The casts should not be handled on the teeth or ridge areas. 10-30. Preparing Refractory Cast for Framework Pattern a. An accurate pattern of the desired framework is formed from wax or plastic preformed patterns on the refractory cast. Later a mold of additional investment material is placed around the pattern and the pattern material is eliminated from the mold by heat. This leaves a void into which molten metal is forced, forming a metal reproduction of the original pattern. b. The refractory cast is placed on a mesh or perforated shelf in a vented dehydrating oven at 180° to 200° F. for 1 hour with the oven door closed (A, fig 10-43). c. When the refractory cast is thoroughly dried, its surface is prepared for waxing by one of the following methods. |